On Tue, Jul 10, 2018 at 03:26:30PM -0700, Yu-cheng Yu wrote:

WRUSS is a new kernel-mode instruction but writes directly
to user shadow stack memory.  This is used to construct
a return address on the shadow stack for the signal
handler.

This instruction can fault if the user shadow stack is
invalid shadow stack memory.  In that case, the kernel does
fixup.

+static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
+{
+	int err = 0;
+
+	asm volatile("1: wrussq %[val], (%[addr])\n"
+		     "xor %[err], %[err]\n"

this XOR is superfluous, you already cleared @err above.

+		     "2:\n"
+		     ".section .fixup,\"ax\"\n"
+		     "3: mov $-1, %[err]; jmp 2b\n"
+		     ".previous\n"
+		     _ASM_EXTABLE(1b, 3b)
+		     : [err] "=a" (err)
+		     : [val] "S" (val), [addr] "D" (addr));
+
+	return err;
+}
+#endif /* CONFIG_X86_INTEL_CET */
+
 #define nop() asm volatile ("nop")

What happened to:

  https://lkml.kernel.org/r/1528729376.4526.0.camel@2b52.sc.intel.com
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